[stdlib] add MutableSpan and MutableRawSpan

Author: glessard

stdlib/public/core/CMakeLists.txt

@@ -157,6 +157,8 @@ split_embedded_sources(
   EMBEDDED Slice.swift
   EMBEDDED SmallString.swift
   EMBEDDED Sort.swift
+  EMBEDDED Span/MutableRawSpan.swift
+  EMBEDDED Span/MutableSpan.swift
   EMBEDDED Span/RawSpan.swift
   EMBEDDED Span/Span.swift
   EMBEDDED StaticString.swift

stdlib/public/core/GroupInfo.json

@@ -200,6 +200,8 @@
     "UnsafeRawBufferPointer.swift"
   ],
   "Span": [
+    "MutableRawSpan.swift",
+    "MutableSpan.swift",
     "RawSpan.swift",
     "Span.swift"
   ],

stdlib/public/core/LifetimeManager.swift

@@ -297,3 +297,22 @@ internal func _overrideLifetime<
   // should be expressed by a builtin that is hidden within the function body.
   dependent
 }
+
+/// Unsafely discard any lifetime dependency on the `dependent` argument.
+/// Return a value identical to `dependent` with a lifetime dependency
+/// on the caller's exclusive borrow scope of the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(borrow source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T,
+  mutating source: inout U
+) -> T {
+  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
+  // should be expressed by a builtin that is hidden within the function body.
+  dependent
+}